Problem: The scale on a map is 7cm : 10km. If the distance between two cities is 30km, how far apart in cm are the two cities on the map?
Solution: The scale means that every 7cm on the map corresponds to an actual distance of 10km. An actual distance of 30km is the same as 3 $\cdot$ 10km. The distance between the two cities on the map is 3 $\cdot$ 7cm, or 21cm.